A wide variety of different types of viruses are well-known as being the etiologic agents for a number of diseases in both animals and man. Because of the large number of potentially widespread epidemics, e.g. influenza, herpes, and AIDS, to name but a few, methods are constantly being sought for either prevention or cure of the diseases caused by these entities. This effort has been hampered to a large extent by the unusual structural and functional aspects of viruses, which are quite unlike any other known infectious agents, such as bacteria or fungi. The virus itself consists essentially of nucleic acid surrounded by a lipid-protein envelope; the virus does not replicate in the host by simple division like a bacterium, but rather multiplies by invading a host cell and, by virture of the action of the viral nucleic acid, reprogramming the cell to synthesize the viral components. The extensive use of mimicry of cellular mechanisms by the virus makes it especially difficult to generate drugs which are selectively toxic to viral infection.
In more recent years, an increased understanding of the structure, and related function of different viruses has provided an insight into the detailed mechanisms by which a viral particle invades a cell. The protein elements of the envelope, which generally consist of matrix proteins and glycoproteins, may play an integral role in the infection process. In fact it is now known that the glycoprotein components of the envelopes of many viruses are absolutely critical to the successful entry of the virus into the host cell. For example, in a large number of essentially unrelated types of virus, such as paramyxoviruses, influenza viruses and retroviruses, a common pattern exists. Attachment or adsorption of the virus to the host cell membrane is achieved by the interaction of an "attachment" or "receptor-binding" viral glycoprotein with a specific receptor on the host cell surface. Following attachment of the virus, fusion of the target cell membrane with the viral envelope occurs via the mediation of a fusion glycoprotein of the virus, which probably penetrates the host cell at a particular site, and then may shorten, drawing the two entities in closer proximity. Once fusion occurs, the cytoplasm of the cell is merged with the contents of the virus and the viral nucleic acid may then begin to direct the cell machinery.
This knowledge of the mechanism of cell invasion by the virus provides a possible key to development of methods of prevention of penetration. It is theoretically possible to attempt to disrupt the process, at any one of the aforementioned steps and therefore prevent the virus from gaining access to the inside of the cell. One way in which this can be done is by blocking the receptor sites of the glycoprotein or otherwise preventing one or both from carrying out the attachment and/or fusion process. This in fact has been achieved in paramyxoviruses, by application of of small peptide, Phe-X-Gly; which mimics the critical binding function of the fusion peptide (Richardson, et al., Virology 105:202-222, 1980; Varsanyi, et al., Virology 147: 110-117, 1985). The peptide homologue somehow interferes with the normal function of the fusion protein, thereby preventing fusion and subsequent infection of the cell by the virus. Although a promising indication, however, there is typically no possibility of extrapolating treatment for one type of virus to other unrelated types: each family of viruses typically is characterized by its own particular glycoproteins, each having a specific length and amino acid sequence; these may, in fact, vary to some extent even within families, or among variants in a "genus." Structural homologues between unrelated or distantly related virus groups are very uncommon. It is this variability in envelope structure, in addition to potential variables in type and arrangement of nucleic acid, which makes successful treatment of viral disease so unpredictable, and also explains the unavailability of broad spectrum antiviral agent. Thus, each group of viruses must be treated separately when considering possible therapeutic regimens.
In this vein, the viral disease currently presenting the greatest concern to the human population is acquired immune deficiency syndrom (AIDS). Now reaching nearly epidemic proportions, the disease has to date evaded all attempts to contrtol or cure it. Relatively little is known about the causative agent, variously referred to as HTLV-III, LAV or HIV (usually HIV-1). It is known to be a retrovirus, a group of viruses characterized by the presence of a single-stranded RNA and reverse transcriptase in the virion. Among the other retroviruses are many oncogenic viruses which induce sarcomas, leukemias, lymphomas, and mammary carcinomas. The AIDS virus, and other retroviruses appear to infect cells by the same attachment-fusion process described above. The product of the env gene of HIV, which codes for the envelope glycoprotein of the virus is apparently unique, and shows no significant homology with any known protein (Wain-Hobson et al., Cell 40: 9-17, 1985). The sequence of the HIV envelope glycoprotein has been described by Muesing et al. (Nature 313:456-458, 1985). However, even with this knowledge of the structure of the env product, there has been, to date, only limited success in exploiting this information for the production of inhibitory compounds. An octapeptide, Peptide T, has been synthesized based on homology to a sequence in gpl20, the attachment glycoprotein, which is purported to inhibit binding of HIV to susceptible cells. The sequence is Ala-Ser-Thr-Thr-Thr-Asn-Tyr-Thr (Pert, et al., PNAS USA 83:9254-9258, 1986). The proposed utility of this peptide has caused much controversy. (Science 237:128-130, 1987).
It has now been unexpectedly discovered that a short sequence, Phe-Leu-Gly-Phe-Leu-Gly, within the fusion glycoprotein (gp41 in HIV, a cleavage product of the larger envelope glycoprotein gpl60) of all retroviruses studied, including HIV, is highly conserved and appears to represent the critical region of the molecule for fusion purposes. Even more surprisingly, this sequence is found to some extent to correspond to similiar sequences in fusion glycoproteins of paramyxoviruses, although the remainder of the glycoprotein molecule sequences are quite dissimiliar between the two groups. Examples of sequences of a number of retroviruses and paramyxoviruses is shown in Table 1 on next page. More importantly, however, it has further been determined that small peptides containing this tripeptide sequence or homologues thereof are capable of inhibiting the fusion process of the AIDS virus and other retroviruses, and thus provides a useful form of therapy and prophylaxis for individuals exposed to retrovirus, particularly HIV-1 infection. TBL3 TABLE 1 MSLS . . . . . . . . . . Phe Ala Gly Val Ile Leu Ala Gly Ala Ala Leu Gyl Val Ala Thr Ala Ala ReSV Arg Arg . . . . . . . . Phe Leu Gly Phe Leu Leu Gly Val Gly Ser Ala Ile Ala Ser Gly Val Ala HIV-1 Lys Arg Ala Val Gly . Ile Gly Ala Leu Phe Leu Gly Phe Leu Gly Ala Ala Gly Ser Thr Met Gly Ala Arg Ser Met Lys Arg Ala Val Gly Ala Ile Gly Ala Met Phe Leu Gly Phe Leu Gly Ala Ala Gly Ser Thr Met Gly Ala Ala Ser Leu Lys Arg Ala Ile Gly . Leu Gly Ala Met Phe Leu Gly Phe Leu Gly Ala Ala Gly Ser Thr Met Gly Ala Ala Ser Leu HIV-2 Thr Arg Gly Val Phe Val Leu Gly . . Phe Leu Gly Phe Leu Ala Thr Ala Gly Ser Ala Val Gly Ala Ala Ser Leu STLV-3 Lys Arg Gly Val Phe Val Leu Gly . . Phe Leu Gly Phe Leu Ala Thr Ala Gly Ser Ala Ile Gly Ala Ala Ser Val Viana Lys Arg Gly Ile Gly Leu Val . . Ile Val Leu Ala Ile Met Ala Ile Ile . Ala Ala . . Gly . Ala Gly SRV-1 Lys Arg Ala Ile Glu Phe Ile . Pro Leu Val Ile Gly . Leu Gly Ile Thr Thr Ala Val Ser Thr Gly Thr Ala Gly Comparison of amino acid sequences of the fusion glycoproteins of a numbe of retroviruses [(HIV1, HIV2, Simian TCell lymphotrophic virus (STLV), Visra virus, and simian retrovirus (SRV1)], and two paramyxoviruses [measles (MSLS) and respiratory syncytral virus [(ResV)].